Over the years various kinds of machines have been developed for cleaning and maintaining floors inside buildings, and paved outdoor areas such as streets, sidewalks and parking lots. They include such machines as rotary broom sweepers, vacuum sweepers, scarifiers, burnishers, polishers and scrubbers. For our purposes here they can be divided into machines which apply water to the surface being maintained and machines which operate dry. We are concerned with the latter, which would include many vacuum sweepers, scarifiers, and rotary broom sweepers. They all share one problem which is addressed by this invention. In their normal operation they tend to stir up dust from the surface being maintained. If it is not controlled, this dust is highly objectionable.
On many of these machines the problem has received one general solution. The functional tool which generates the dust, such as a rotary broom, a scarifier head, or a vacuum pickup, is provided with a cover and surrounded by walls which have rubber skirts that hang down almost to the surface being maintained. An on board exhaust blower continuously pulls air from the tool chamber thus created so there is a sub-atmospheric air pressure within it which eliminates outflow of dusty air from under the skirts. The blower exhausts this air to atmosphere. One or more air filters are placed in this air path, either upstream or downstream from the blower, to remove dust from the air before it is released so the discharge to atmosphere will be dust free.
The dust thus removed from the air stream builds up on the filters and in time will block off the airflow through them unless they are periodically cleaned, so a cleaning means is commonly provided which the machine operator must use when needed. Generally the operator must be watchful for dust coming out from under the skirts, which indicates that airflow has been reduced by dust plugging the filters. He or she then must stop the machine, shut off the air flow, and activate the cleaning means. This is most commonly a mechanism that shakes or jolts the filters for a predetermined time to shake off the dust, which falls to a collection tray for later removal. Then the air flow is re-started and the machine operation is resumed. This method is effective when properly carried out, but it takes time away from productive operation. Some operators carelessly neglect doing it when needed, so that dust escapes from the machine while it is operating and settles on objects in the area. Also, filters that are allowed to get too dirty cannot be as effectively cleaned and must be replaced more often than properly serviced filters.
To eliminate these shortcomings it has been desirable to provide an automatic filter cleaning system that requires no attention from the operator. There have been efforts in that direction. U.S. Pat. Nos. 4,637,825 and 4,756,727 each show a sweeper filter that is automatically cleaned while the sweeper is operating by directing pulses of compressed air sequentially against segments of the filter from its clean side to blow off dust. U.S. Pat. No. 5,013,333, incorporated by reference herein, shows a sweeper filter that is automatically cleaned while the sweeper is operating by application of periodic activation of a mechanical shaker mechanism.